Generally, in a cellular radiocommunication system, a coverage area is divided into smaller sub-areas known as cells, where each cell is assigned a set of radio resources for serving subscribers. In a FDMA (Frequency Division Multiple Access) cellular system, the radio resources comprise multiplexed radio frequency channels. In a TDMA system (Time Division Multiple Access), the information is transmitted during assigned time slots of a set of radio frequency channels, with different users being assigned corresponding time slots on the same or different radio frequency channels. In both the FDMA and TDMA cellular systems, however, the assignment of the radio frequency channels between the cells produces two types of interference: co-channel interference and adjacent channel interference. Co-channel interference results if two separate signals are sent on the same frequency simultaneously, thereby limiting the ability of a receiver receiving both signals to separate them for detection of the desired signal. Adjacent channel interference results if channels that are adjacent to each other in the frequency spectrum are used in the same geographical area. Transmissions on a radio frequency channel tend to leak to adjacent frequencies and cause interference.
Conventionally, co-channel interference and adjacent channel interference in cellular systems are reduced by intelligently dividing the radio frequency resources among the cells. Most cellular systems use a predefined frequency re-use pattern that is optimized based on a particular characteristic of a coverage area. Under such frequency re-use pattern, adjacent cells are assigned different radio frequency channels, with the same radio frequency channel assignment being repeated for distant cells. Because some of the radio frequency channels are re-used in distant cells, co-channel interference can not be totally eliminated.
Improved frequency planning that relies on cell relations can further reduce co-channel. The cell relations among the cells is often expressed by statistically estimating measured cell relation parameters. Known methods for estimating cell relations have focused on measuring overall interference in a certain cell. Such measurements are usually done, while ordinary communication traffic is in progress. Under such circumstances, it is often difficult to identify the source of a signal for measurement purposes, since the radio frequency channel over which measurements are made may include contributions from several sources. It is thus difficult to accurately measure how much transmissions in one cell interferes with transmissions in another cell, if such transmissions occurs simultaneously on the same frequency. As a result, conventional methods do not provide an accurate representation of cell relations among the cells.
It is, therefore, desirable to accurately measure cell relations, preferably, in both the up-link direction and downlink direction, without interfering with ongoing traffic.